This invention relates generally to flowmeters of the vortex type, and more particularly to an adapter cap for the shedding body of a flowmeter acting to reduce the effective area of the fluid passing by the body without altering the basic geometry of the body, thereby making it possible to measure flow rates below the normal operating range of the meter.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate, this phenomenon being exploited to create a flowmeter. Flowmeters of the vortex-shedding type are disclosed in U.S. Pat. Nos. 3,116,639 and No. 3,572,117, among others.
U.S. Pat. No. 3,589,185 describes an improved form of vortex-type flowmeter wherein the signal derived from the fluidic oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio, thereby insuring accurate flow-rate information over a broad range. In this meter, an obstacle assembly is mounted in the flow tube, the assembly being constituted by a block positioned across the tube with its longitudinal axis at right angles to the direction of fluid flow, a strip being similarly mounted across the conduit behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This street is sensed by a pressure or other form of transducer to produce an electrical signal whose frequency is proportional to flow rate.
A conventional vortex-shedding flowmeter has a fixed metering range within which it is capable of accurately measuring flow rate. This range is largely determined by linearity requirements, signal recovery parameters and internal velocity limitations.
In some instances, it becomes necessary to accurately measure low flow velocities which lie below the normal operating range of a standard vortex-type meter. With existing meters, it is not possible, in the field, to alter the operating range of an installed meter. Moreover, it is difficult to produce a small capacity flowmeter; for with existing vortex-shedding meter structures, the vortex-sensing system cannot economically be miniaturized.
In U.S. Pat. No. 4,003,253 to Yard et al., there is disclosed an adapter for a standard vortex-shedding flowmeter. This adapter, when applied to the meter, renders it capable of measuring low flow velocities below the normal operating range thereof, the adapter acting to restrict the effective area of fluid traversing the vortex-shedding body, whereby for the same velocity of flow past the shedding body, a smaller amount of fluid is metered.
A significant feature of the Yard et al. invention is that by means of adapters of different size, one can readily change the operating range of the meter in the field without, however, adversely affecting the linearity of the meter.
In the Yard et al. arrangement, the adapter is constituted by a pair of rods which are inserted longitudinally into the flow tube on opposite sides of the vortex-shedding block mounted transversely across the tube. These rods act to restrict the effective area of the fluid traversing the block whereby for the same velocity of flow past the block, a smaller amount of fluid is metered.
The Yard et al. arrangement presents certain practical difficulties. Thus there is the problem of supporting the adapter rods within the flow tube so that they conform to the internal wall of the flow tube and yet can be readily withdrawn from the tube. Second, there is the problem raised by the shape of the shedding body, for the inserted rods are only effective with certain shedder shapes.